The present invention relates to a compressed gas aerosol spray system which conserves gas yet retains effective spray characteristics by disposing a vapor tap hole in the dip tube below the initial liquid level.
Aerosol spray systems have achieved a large presence in the market place due to their ease of application and the wide variety of products which can be applied via airborne particles. For example, furniture polishes, air fresheners, insecticides, paints, deodorants, etc. can be readily and effectively applied using an aerosol spray system. As is known, aerosol spray systems for such products use hydrocarbon propellants to great advantage. Specifically, these propellants are liquefiable and may be maintained in the aerosol spray container in both the liquid and gaseous states. As the liquid product is emptied from the container, more of the propellant vaporizes, thus retaining relatively high pressure within the container to ensure that the desirable spray characteristics remain relatively constant until the container is emptied. However, such hydrocarbon propellants can produce by-products which are detrimental to the environment and are flammable.
The use of a compressed gas such as nitrogen or air to charge an aerosol spray device would effectively overcome the possible problems inherent in the use of hydrocarbon propellants in that they are safe and nonflammable. However, such compressed gases are not readily liquefiable. Therefore, the pressure within the container is reduced as the liquid product is emptied therefrom. Without sufficient pressure inside the container, an effective aerosol spray cannot be achieved since the spray characteristics (such as spray cone angle, spray pattern shape, particle size, etc.) of the spray will deteriorate with falling pressure. This is because the aerosol spray is produced by mechanical breakup of the liquid at high pressures, for example, at 80-125 pounds per square inch ("psi") which is 0.552-0.862 MegaPascal ("MPa"). Below such pressures, anomalies such as increased particle size, streaming, reduced spray cone angle, etc. may occur, and the spray characteristics become unsatisfactory.
FIG. 1 depicts one potential problem of using compressed gas in a typical aerosol spray system. In such an aerosol spray system, for example, a mixture of 60% by volume liquid and 40% by volume compressed nitrogen may be pressurized in a container to e.g., 120 psi (0.827 MPa) and may initially produce a particle spray size of 50 microns. As shown by the continuous line in FIG. 1, as the liquid is emitted from the container, the pressure will drop and the particle size will tend to increase The intermittent line shows the condition of the prior art system wherein the particle size eventually may reach 90 microns at 80 psi (0.551 MPa) pressure within the container. At that point the spray characteristics will deteriorate and the spray may become much heavier with large-sized droplets. It can be readily seen that when the pressure in the container is eventually reduced to approximately 20 psi (0.138 MPa), a mere stream of liquid may be emitted from the valve. Thus, the system no longer produces an aerosol spray and is not effective for its intended use. This problem becomes extreme for products such as air fresheners which require a maximum of a 30-50 micron particle size to keep the particles airborne.
This problem cannot be solved by merely increasing the initial pressure within the spray can to 150 psi (1.03 MPa) or greater. A can sturdy enough to withstand such a high initial pressure would also be very heavy, cumbersome to use, and expensive. Furthermore, federal and local transportation regulations limit the amount of pressure for a given container. For example, the U.S. Department of Transportation requires that common aerosol spray cans not exceed pressures of 140 psi (0.965 MPa) at 130.degree. F. (54.4.degree. C.). Therefore, although compressed gas appears to be a good propellant in terms of minimizing damage to the environment, its properties are not well suited for typical aerosol spray cans.
The prior art is replete with structures for emitting various combinations of fluid and gas in an aerosol spray. For example, U.S. Pat. No. 3,260,421 to Rabussier discloses a dispensing device for aerosol pressure containers intended for properly mixing and spraying two immiscible liquids. The push-button valve allows pressurized liquids to enter a plurality of holes in the dip tube and be emitted from the container. The dip tube holes are disposed through both of the liquid phases so that both of the liquids enter the tube and are mixed together therein. To enhance mixing, the push-button valve has a vapor tap orifice leading to the gas volume within the container. Pressurized gas enters the valve through this vapor tap orifice and enhances the mixing and mechanical breakup of the liquids in the valve. A problem with this structure is that pressure is lost because of the vapor tap orifice in the valve. In addition, dip tube holes are also disposed above the liquid level, thus allowing another escape path for the gas.
U.S. Pat. No. 3,184,118 to Webster discloses a aerosol spray container for mixing paint, in which the dip tube has a plurality of small holes in the bottom thereof to screen out large debris which may clog up the valve. However, if compressed gas were used in the container of Webster, the gradual reduction in pressure would result in a liquid paint stream being emitted rather than a paint spray.
U.S. Pat. No. 3,656,657 to Smith et al. discloses apparatus for dispensing fluid mixtures in uniform proportions from pressure containers. The bottom of the dip tube is closed but a liquid entrance hole is located adjacent the closed end. At least one hole is provided in the dip tube below the liquid level, and the container is pressurized to approximately 345 psi (2.38 MPa). Smith et al. does not disclose a specific valve mechanism. Further, it should be appreciated that such large internal pressures would be totally inappropriate for the consumer aerosol spray market.
U.S. Pat. No. 3,129,855 to Malakoff et al. discloses an aerosol package which provides an emergency reserve of sprayable liquid material. When the level of the liquid in the container falls below the level of the openings in the dip tube, only gas is discharged. The user then tips the aerosol container to gain access to the reserve liquid. Again, if compressed gas were utilized in this aerosol package, the decrease in pressure would produce a very large particle size.
Thus, it is an object of this invention to provide an aerosol spray system using compressed gas but which provides satisfactory spray characteristics after partial discharge to achieve a safe, environmentally benign, reliable aerosol spray.